1. Field of the Invention
The present invention relates to a liquid crystal display apparatus including a liquid crystal panel using a polarizer layer with improved viewing angle characteristics.
2. Description of the Related Art
A cathode ray tube (CRT, also generally referred to as “Braun tube”) has been a mainstream of display devices, but liquid crystal displays have the benefit of being less thick and more lightweight than CRTs. Further, with the advance of the technology for improving image quality of the liquid crystal displays, the use and market for the liquid crystal displays have been expanding.
Recently, the liquid crystal displays have been used in monitors for desktop personal computers, monitors for printing or designing, or the like. Demands have been increasing to provide better color reproducibility and higher contrast ratio with broadening applications as liquid crystal televisions. In particular, the viewing angle characteristics in contrast ratio are very important in the liquid crystal televisions which become increasingly popular along with digital broadcasting and high-definition broadcasting. With respect to the viewing angle characteristics, not only a specific angle in a case where the liquid crystal display is viewed in an oblique direction is taken into account. For example, when a normal liquid crystal television set having an aspect ratio (length-to-width) of 9:16 is observed at a distance of 3H (three times of a screen longitudinal length) recommended for high-definition broadcasting reception, an observer observes display information with an angle of at least 33 degrees (plus/minus 16.5 degrees in lateral direction).
When an observer does not watch the display in a center direction, an asymmetrically larger angle can come into view. When the display information is observed by the observer at a distance of 2H to obtain more realism, the screen is viewed at a viewing angle of 48 degrees. That is, when a contrast ratio is remarkably changed depending on the viewing angle, the observer views the screen with significantly different image qualities. Therefore, it is likely to cause not only uncomfortable feeling about the screen but also eyestrain.
The viewing angle characteristics of the liquid crystal display apparatuses depend on the viewing angle characteristics of polarizers including a stretched component of iodine or a dichroic dye, and the viewing angle characteristics of liquid crystal layers. In order to improve the viewing angle characteristics, an optical film exhibiting a phase difference is generally used. For example, JP 2005-128498 discloses a horizontal electric field mode liquid crystal display apparatus using a retarder film.
Two polarizers obtained by dyeing polyvinyl alcohol with iodine and then stretching the dyed polyvinyl alcohol are used for a normal liquid crystal display apparatus. The degree of polarization of a stretched polarizer is sufficiently high. For example, in many cases, a contrast ratio of the polarizers (the contrast ratio is obtained by dividing brightness in case where a pair of polarizers are arranged to have polarized axes parallel to each other into brightness in case where the two polarizers are arranged to have polarized axes orthogonal to each other) is equal to or larger than 10,000.
However, a contrast ratio of the liquid crystal display apparatus using the two polarizers, which is observed by observers, is as low as approximately 1/10 of the contrast ratio of the polarizers. This is because a member which converts incident polarized light into light having an undesirable polarized state or a partial depolarization state is located in a liquid crystal panel provided between the pair of polarizers.
An iodine stretched polarizer which is normally used has extremely low polarizance to light incident from an oblique direction. In the liquid crystal display apparatus, light from a back light is incident on the liquid crystal panel from substantially all directions including a polar angle and an azimuthal angle. The characteristic in which the degree of polarization of the polarizer in the oblique direction is low causes a large reduction in contrast ratio in the oblique direction. As a result, the contrast ratio of the liquid crystal display apparatus in the oblique direction reduces to as low as approximately 10.
According to JP 2005-128498 A, means for improving a viewing angle characteristic with respect to a contrast ratio using a retarder film, that is, means for reducing light leakage in the oblique direction during a black state is disclosed. The retarder film changes a phase of incident polarized light because of the birefringence, but a refractive index value largely depends on a wavelength. For example, when a design value capable of compensation of viewing angle characteristics at a wavelength of 550 nm (green) is used, complete compensation cannot be achieved in wavelength regions corresponding to blue (equal to or larger than 430 nm and equal to or smaller than 480 nm) and red (equal to or larger than 600 nm and equal to or smaller than 700 nm), which are wavelengths separate from the wavelength of 550 nm, thereby causing light leakage. Therefore, cyan or magenta tint occurs. This degrades image quality in a black state.